This invention relates generally to projection displays.
A projection display typically includes one or more spatial light modulators that modulate light to produce a projected image. The spatial light modulator may include, for example, a liquid crystal display such as a high temperature polysilicon (HTPS) liquid crystal display panel or a liquid crystal-on-silicon microdisplay, a grating light valve, or a microelectromechanical device light modulator such as a digital mirror display to modulate light that originates from a lamp of the projection display system. In typical projection displays, the light output is formatted with optics to deliver a uniform level on a surface of the spatial light modulator. The spatial light modulator forms a pictorial image by modulating the illumination into spatially distinct tones ranging from dark to bright, based on the supplied video data. Additional optics then relay and magnify the modulated illumination pattern onto a screen for viewing.
The spatial light modulator typically includes an array of pixel cells, which are electrically controllable to establish the intensity of a pixel of the projected image. In some projection displays, spatial light modulators are transmissive and in others they are reflective. In a digital drive scheme, a spatial light modulator may be operated so that each pixel has only two states. A default reflective state causes either a bright or a dark projected pixel and a non-default reflective state causes the opposite projected pixel intensity. In the case of a liquid crystal-on-silicon spatial light modulator, the pre-alignment orientation of the liquid crystal material and any retarders in the system determine whether the default reflective state is normally bright or normally dark. Thus, in its basic operation, the pixel cell may be digitally controlled to form either a dark pixel (for example, in a non-default reflective state) or a bright pixel (for example, in a default reflective state). In an analog drive scheme, on the other hand, the level of reflection or transmission from the spatial light modulator may be incrementally controlled by adjusting the applied voltage level.
In order to produce each of the primary colors from a display that uses a single spatial light modulator (also termed as imager or panel), a color wheel may be utilized. A color wheel typically is a segmented color filter wheel which rotates through an illumination beam produced by a light source. The color wheel has three or more pie-shaped regions consisting of color filters that transmit each of the primary colors. Thus, for example, as the color wheel rotates, it progressively forms a stripe of red across the entire spatial light modulator, then a stripe of green across the entire spatial light modulator, and then a stripe of blue across the entire structure. Thus, in a time multiplex fashion, the spatial light modulator progressively produces red, green, and blue images in the example just given, wherein the sequence is red, then green, then blue.
One problem with such an approach is that when red light is being transmitted to the spatial light modulator, green and blue light is being discarded. In such case, about two-thirds of the light intensity is effectively discarded. Necessarily, this means that brighter illumination sources are needed.
The reduced brightness efficiency of color wheel systems necessitates higher wattage lamps. Higher wattage lamps have shorter lifetimes and higher costs. As an example, currently, 100 Watt lamps reach about 10,000 hours of life, while 150 Watt lamps are limited to only about 6,000 hours of life. Thus, projection systems using a 150 Watt or higher lamp will require the consumer to change the light bulb. Usage of higher wattage lamps naturally also leads to more power consumption.
Thus, there is a need for better ways to more effectively use the available illumination.